Complete Guide to Using the Correct Charger or Power Adapter (and What Happens If You Don’t)

The other weekend, I sat down and sorted through all my random electronics junk. As part of that process, I took all my power supplies and adapters and threw them into a box. It ended up being a pretty big box. I’m willing to bet that any given household has a dozen or more types of cell phone chargers, AC/DC adapters, power bricks, power cables, charger plugs, or whatever you want to call those things that plug into the wall on one end and then plug into a gadget on the other end.

Having so many chargers can be pretty frustrating. It’s easy to get them separated from the phone or laptop or tablet or router or whatever they belong to. And once that happens, it can be incredibly difficult to figure out which goes with which. The default solution to this is to simply try random plugs until you find one that fits into your device.  This, however, is a big gamble. If you grab an incompatible power adapter, your best case scenario is that it works, albeit not the way the manufacturer intended. The second worst case scenario is that you fry the gadget you are trying to power up. The worst case scenario is that you burn down your house.

In this article, I’m going to walk you through the process of digging through your junk drawer and finding the right power adapter for your device. Then, I’ll tell you why it’s so important to do so.

In a nutshell:

• The following will cause damage to your device:
  • Reverse polarity
  • Higher voltage adapter than device rating
• The following will cause damage to your power cord or adapter:
  • Reverse polarity
  • Lower current adapter than device rating
• The following might not cause damage, but the device will not work properly:
  • Lower voltage adapter than device rating
  • Higher current adapter than device rating

A Very Brief Introduction to Electrical Terminology

Each AC/DC power adapter is specifically designed to accept a certain AC input (usually the standard output from a 120 V AC outlet in your home) and convert it to a specific DC output. Likewise, each electronic device is specifically designed to accept a certain DC input. The key is to match the DC output of the adapter to the DC input of your device. Determining the outputs and inputs on your adapters and devices is the hard part.

Power adapters are a bit like canned food. Some manufacturers put a lot of information on the label. Others put just a few details. And if there is no information on the label, proceed with extreme caution.

The most important details for you and your delicate electronics are the voltage and thecurrent. Voltage is measured in volts (V) and current is measured in amps (A).  (You’ve probably also heard about resistance (Ω), but this doesn’t usually show up on power adapters.)

To understand what these three terms mean, it helps to think of electricity as water flowing through a pipe. In this analogy, the voltage would be like the water pressure. Current, as the term implies, refers to the flow rate. And resistance refers to the size of the pipe. Tweaking any of these three variables increases or decreases the amount of electrical power that’s delivered to your device. This is important because too little power means your device won’t charge or operate correctly. Too much power generates excess heat, which is the bane of sensitive electronics.

The other important term to know is polarity. For direct currents, there is a positive pole (+) and a negative pole (-). For an adapter to work, the positive plug must mate with a negative receptacle or vice versa. Direct current, by nature, is a one-way street, and things just won’t work if you try to go up the downspout.

If you multiply the voltage by the current, you get the wattage. But the number of watts alone won’t tell you if the adapter is right for your device.

Reading an AC/DC Adapter Label

If the manufacturer was smart enough (or compelled by law) to include the DC output on the label, you are in luck. Look on the “brick” part of the adapter for the word OUTPUT. Here, you’ll see the volts followed by the direct current symbol and then the current.

The DC symbol looks like this: 

To check the polarity, look for a + or – sign next to the voltage. Or, look for a diagram showing the polarity. It will usually consist of three circles, with a plus or minus on either side and a solid circle or C in the middle. If the + sign is on the right, then the adapter has positive polarity:

If there is a – sign on the right, then it has negative polarity:

Next, you want to look at your device for the DC input. You’ll usually see at least the voltage near the DC plug receptacle. But you also want to make sure the current matches, too.

You might find both the voltage and the current elsewhere on the device, on the bottom or inside a battery compartment cover or in the manual. Again, look for the polarity, by either noting a + or – symbol or the polarity diagram.

Remember: the input of the device should be the same as the output of the adapter. This includes polarity. If the device has a DC input of +12V / 5.4A, get an adapter that has a DC output of +12V / 5.4A. If you have a universal adapter, make sure it has the proper current rating and that you choose the correct voltage and polarity.

Fudging It: What Happens if You Use the Wrong Adapter?

Ideally, you’ll have the same voltage, current and polarity on your adapter and device.

But what if you accidentally (or purposefully) use the wrong adapter? In some cases, the plug won’t fit. But there are many cases where an incompatible power adapter will plug into your device. Here’s what you can expect in each scenario:

• Wrong polarity – If you reverse the polarity, a few things can happen. If you’re lucky, nothing will happen and no damage will occur. If you are unlucky, your device will be damaged. There’s a middle ground, too. Some laptops and other devices include polarity protection, which is essentially a fuse that burns out if you use the wrong polarity. If this happens, you might hear a pop and see smoke. But the device may still work on battery power. However, your DC input will be toast. To fix this, either replace the polarity protection fuse or get it serviced. The good news is that the main circuitry wasn’t fried.
• Voltage too low – If the voltage on an adapter is lower than the device, but the current is the same, then the device may work, albeit erratically. If we think back to our analogy of voltage being water pressure, then it would mean that the  device has “low blood pressure.”  The effect of low voltage depends on the complexity of the device. A speaker, for example, may be fine, but it just won’t get as loud. More complex devices will falter, and may even shut themselves off when they detect an under-voltage condition. Usually, an under-voltage condition won’t cause damage or shorten the life of your device.
• Voltage too high – If the adapter has a higher voltage, but the current is the same, then the device will likely shut itself off when it detects an overvoltage. If it doesn’t, then it may run hotter than normal, which can shorten the life of the device or cause immediate damage.
• Current too high – If the adapter has the correct voltage, but the current is higher than what the device input requires, then you shouldn’t see any problems. For example, if you have a laptop that calls for a 19V / 5A DC input, but you use a 19V / 8A DC adapter, your laptop will still get the 19V voltage it requires, but it will only draw 5A of current. As far as current goes, the device calls the shots, and the adapter will have to do less work.
• Current too low – If the adapter has the correct voltage, but the adapter’s rated current is lower than what the device input, then a few things might happen. The device could power on, and simply draw more current from the adapter than it’s designed for. This could cause the adapter to overheat or fail. Or, the device may power on, but the adapter may not be able to keep up, causing the voltage to drop (see voltage too low above). For laptops running on undercurrent adapters, you might see the battery charge, but the laptop is not powering on, or it may run from power, but the battery won’t charge. Bottom-line: it’s not a good idea to use a lower current rating adapter since it could cause excess heat.

All of the above are what you would expect to see, based on a simple understanding of polarity, voltage and current. What these outlooks don’t take into account is the various protections and versatility of adapters and devices. Manufacturers may also build a bit of a cushion into their ratings. For example, your laptop may be rated for an 8A draw, but in reality, it only draws around 5A. Conversely, an adapter may be rated at 5A, but in reality, can withstand currents up to 8A. Also, some adapters and devices will have voltage and current switching or detecting features which will adjust the output/draw depending on what’s needed. And, as mentioned above, many devices will automatically shut down before damage is caused.

That being said, I don’t recommend fudging the margin under the assumption that you can do the equivalent of driving 5 MPH over the speed limit with your electronic devices. The margin is there for a reason, and the more complicated the device, the more potential for something to go wrong.

Have any cautionary tales about using the wrong AC/DC adapter? Warn us in the comments!

P.S. Wall adapters that give you a USB port for charging aren’t nearly as tricky. Standard USB devices have a voltage of 5 V dc and a current up .5 A or 500 mA for charging only. This is what allows them to play nice with the USB ports on your computer. Most USB wall adapters will be 5 V adapters and have a current rating well over .5 A. The iPhone USB wall adapter I’m holding in my hand right now is 5 V / 1 A. You also don’t have to worry about polarity with USB. A USB plug is a USB plug, and all you usually have to worry about is form factor (e.g. micro, mini or standard). Furthermore, USB devices are usually smart enough to shut things down if something isn’t right. Hence, the oft-encountered “Charging is not supported with this accessory” message.